1. Field of the Invention
This invention relates to a small variable focal length lens system with a small length which is adapted for a lens shutter camera and the like.
2. Description of the Related Art
Recently variable focal length lenses have been widely used as the taking lenses of lens shutter cameras, and consequently smaller variable focal length lenses have been demanded.
Conventional variable focal length lenses used for lens shutter cameras which have a zooming ratio of about 2 are mostly two-unit zoom lenses comprising first lens units of positive refractive power and second lens units of negative refractive power. In this type of zoom lens, however, the tele ratio at the wide position is at most about 1.3 and the amount of shift of the respective units at the time of zooming is large, thus the total length of the lens system is large and it is difficult to reduce the thickness of the camera. Furthermore, while the total length of the two-unit zoom lens can be reduced by increasing the refractive power of the respective units, then the variation of aberrations due to zooming is large and difficult to correct well.
On the other hand, the three-unit zoom lens comprising a first lens unit of positive refractive power, a second lens unit of positive refractive power and a third lens unit of negative refractive power can be made small with aberrations well maintained by setting the power of the respective units properly. Examples of prior art using this type of three-unit zoom are as follows: as examples of a zooming ratio of about 1.5, Japanese Laid-Open Paten Applications, Publication Nos. Sho 60-263113 and Sho 62-78522; and as examples of a zooming ratio of about 2, Japanese Laid-Open Patent Applications, Publication Nos. Hei 1-93713 and Hei 2-50117.
However, each of these conventional zoom lenses has a tele ratio o about 1.3 at the wide position and is as large as a two-unit zoom lens. The zoom lenses disclosed in Japanese Laid-Open Patent Applications, Publication Nos. Sho 62-78522, Hei 1-93713 and Hei 2-50117 comprise a large number of lens components and are disadvantageous in terms of manufacturing costs. While the zoom lens disclosed in japanese Laid-Open Patent Application, Publication No. Sho 60-263113 comprises a small number of lens components, its zooming ratio is small, i.e., 1.5 and the level of correction of aberrations is not satisfactory for practical use. Similarly, the level of correction of aberrations in Japanese Laid-Open Patent Application, Publication No. Hei 1-93713 is not satisfactory.